1. Field of the Invention
This invention relates to a clustering disk controller and its load balancing method, and in particular to a disk subsystem which stores data in plural magnetic disk units.
2. Description of the Related Prior Art
There is a disk control unit (hereafter, DKC) which performs storage and read-out of data to plural magnetic disk units (hereafter, drives), the drive and DKC together being referred to as a disk subsystem.
One of the requisites of a disk subsystem is enhancement of storage capacity and curtailment of management costs. As there is a limit to the capacity of the drives which can be managed by one DKC, plural disk subsystems are provided to increase the storage capacity. However, the management cost likewise increases. Attention has therefore been focused on a storage area network (hereafter, SAN) which achieves centralization of disk subsystems that were previously connected to each server and therefore dispersed. An example of a type of disk subsystem in a SAN environment is shown in FIG. 2. Plural disk subsystems 1 are connected to a host computer 0 via a SAN switch 39. One disk subsystem comprises only one disk controller 10 (abbreviated to DKC 10 hereinafter), and is connected with the SAN switch 39 via a channel 2. A logical disk 7 is a storage region recognized by a host computer 0. The host computer 0 looks up data in a specific address of the logical disk 7 via the SAN switch 39 and channel 2, and issues update requests. The channel 2 maybe a fiber channel or SCSI, etc. Plural disk control units 10 and drives 17 are connected by a drive IF 16. The drive IF 16 may be a fiber channel or SCSI, etc. A DKC 10 broadly comprises a channel control unit 11 which controls the channels, a disk control unit 14 which controls the drives, a shared memory unit part 12 which stores control information 3 of the DKC, a cache memory unit 13 holding cache data 5, and a connection 15 which connects these component units together. The connection 15 is a bus or an interconnection, etc. The DKC 10 looks up data and performs update processes according to commands from the host computer 0.
However, in a SAN environment, if the host computer does not know in which disk subsystem the data can be accessed, it cannot access the data. Therefore, there is a problem in that the user has to manage the whereabouts of the data. JP-A-2000-99281 discloses an enlarged disk subsystem wherein plural disk subsystems, which conventionally each comprised one DKC, are clustered together without passing via the SAN switch to increase storage capacity and connection channels. The clustering disk subsystem can be managed as one disk subsystem, and therefore can cut down on management costs.
An example of the structure of a clustering disk subsystem is shown in FIG. 3. In the disk subsystem of FIG. 3, mutual data access between plural DKC 10 is made possible by providing a disk controller connecting means 20. This permits sharing of the data between plural disk control units.